The chewate effect is de enhanced affinity of chewating wigands for a metaw ion compared to de affinity of a cowwection of simiwar nonchewating (monodentate) wigands for de same metaw.

The dermodynamic principwes underpinning de chewate effect are iwwustrated by de contrasting affinities of copper(II) for edywenediamine (en) vs. medywamine.

Cu2+ + en ⇌ [Cu(en)]2+

(1)

Cu2+ + 2 MeNH2 ⇌ [Cu(MeNH2)2]2+

(2)

In (1) de edywenediamine forms a chewate compwex wif de copper ion, uh-hah-hah-hah. Chewation resuwts in de formation of a five-membered CuC2N2 ring. In (2) de bidentate wigand is repwaced by two monodentate medywamine wigands of approximatewy de same donor power, indicating dat de Cu–N bonds are approximatewy de same in de two reactions.

The dermodynamic approach to describing de chewate effect considers de eqwiwibrium constant for de reaction: de warger de eqwiwibrium constant, de higher de concentration of de compwex.

[Cu(en)] = β11[Cu][en]

(3)

[Cu(MeNH2)2] = β12[Cu][MeNH2]2

(4)

Ewectricaw charges have been omitted for simpwicity of notation, uh-hah-hah-hah. The sqware brackets indicate concentration, and de subscripts to de stabiwity constants, β, indicate de stoichiometry of de compwex. When de anawyticaw concentration of medywamine is twice dat of edywenediamine and de concentration of copper is de same in bof reactions, de concentration [Cu(en)] is much higher dan de concentration [Cu(MeNH2)2] because β11 ≫ β12.

where R is de gas constant and T is de temperature in kewvins. ΔH⊖{\dispwaystywe \Dewta H^{\ominus }} is de standard endawpy change of de reaction and ΔS⊖{\dispwaystywe \Dewta S^{\ominus }} is de standard entropy change.

Since de endawpy shouwd be approximatewy de same for de two reactions, de difference between de two stabiwity constants is due to de effects of entropy. In eqwation (1) dere are two particwes on de weft and one on de right, whereas in eqwation (2) dere are dree particwes on de weft and one on de right. This difference means dat wess entropy of disorder is wost when de chewate compwex is formed wif bidentate wigand dan when de compwex wif monodentate wigands is formed. This is one of de factors contributing to de entropy difference. Oder factors incwude sowvation changes and ring formation, uh-hah-hah-hah. Some experimentaw data to iwwustrate de effect are shown in de fowwowing tabwe.[3]

These data confirm dat de endawpy changes are approximatewy eqwaw for de two reactions and dat de main reason for de greater stabiwity of de chewate compwex is de entropy term, which is much wess unfavorabwe. In generaw it is difficuwt to account precisewy for dermodynamic vawues in terms of changes in sowution at de mowecuwar wevew, but it is cwear dat de chewate effect is predominantwy an effect of entropy.

Oder expwanations, incwuding dat of Schwarzenbach,[4] are discussed in Greenwood and Earnshaw (woc.cit).

Numerous biomowecuwes exhibit de abiwity to dissowve certain metaw cations. Thus, proteins, powysaccharides, and powynucweic acids are excewwent powydentate wigands for many metaw ions. Organic compounds such as de amino acids gwutamic acid and histidine, organic diacids such as mawate, and powypeptides such as phytochewatin are awso typicaw chewators. In addition to dese adventitious chewators, severaw biomowecuwes are specificawwy produced to bind certain metaws (see next section).[5][6][7][8]

Virtuawwy aww metawwoenzymes feature metaws dat are chewated, usuawwy to peptides or cofactors and prosdetic groups.[8] Such chewating agents incwude de porphyrin rings in hemogwobin and chworophyww. Many microbiaw species produce water-sowubwe pigments dat serve as chewating agents, termed siderophores. For exampwe, species of Pseudomonas are known to secrete pyochewin and pyoverdine dat bind iron, uh-hah-hah-hah. Enterobactin, produced by E. cowi, is de strongest chewating agent known, uh-hah-hah-hah. The marine mussews use metaw chewation esp. Fe3+ chewation wif de Dopa residues in mussew foot protein-1 to improve de strengf of de dreads dat it uses to secure itsewf to surfaces.[9][10][11]

In de 1960s, scientists devewoped de concept of chewating a metaw ion prior to feeding de ewement to de animaw. They bewieved dat dis wouwd create a neutraw compound, protecting de mineraw from being compwexed wif insowubwe sawts widin de stomach, which wouwd render de metaw unavaiwabwe for absorption, uh-hah-hah-hah. Amino acids, being effective metaw binders, were chosen as de prospective wigands, and research was conducted on de metaw–amino acid combinations. The research supported dat de metaw–amino acid chewates were abwe to enhance mineraw absorption, uh-hah-hah-hah.[citation needed]

During dis period, syndetic chewates such as edywenediaminetetraacetic acid (EDTA) were being devewoped. These appwied de same concept of chewation and did create chewated compounds; but dese syndetics were too stabwe and not nutritionawwy viabwe. If de mineraw was taken from de EDTA wigand, de wigand couwd not be used by de body and wouwd be expewwed. During de expuwsion process de EDTA wigand randomwy chewated and stripped anoder mineraw from de body.[14]

According to de Association of American Feed Controw Officiaws (AAFCO), a metaw–amino acid chewate is defined as de product resuwting from de reaction of metaw ions from a sowubwe metaw sawt wif amino acids, wif a mowe ratio in de range of 1–3 (preferabwy 2) mowes of amino acids for one mowe of metaw.[citation needed] The average weight of de hydrowyzed amino acids must be approximatewy 150 and de resuwting mowecuwar weight of de chewate must not exceed 800 Da.[citation needed]

Since de earwy devewopment of dese compounds, much more research has been conducted, and has been appwied to human nutrition products in a simiwar manner to de animaw nutrition experiments dat pioneered de technowogy. Ferrous bis-gwycinate is an exampwe of one of dese compounds dat has been devewoped for human nutrition, uh-hah-hah-hah.[15]

First-generation dentin adhesives were first designed and produced in de 1950s. These systems were based on a co-monomer chewate wif cawcium on de surface of de toof and generated very weak water resistance chemicaw bonding (2–3 MPa).[16]

Chewation derapy is an antidote for poisoning by mercury, arsenic, and wead. Chewating agents convert dese metaw ions into a chemicawwy and biochemicawwy inert form dat can be excreted. Chewation using cawcium disodium EDTA has been approved by de U.S. Food and Drug Administration (FDA) for serious cases of wead poisoning. It is not approved for treating "heavy metaw toxicity".[17]

Awdough beneficiaw in cases of serious wead poisoning, use of disodium EDTA (edetate disodium) instead of cawcium disodium EDTA has resuwted in fatawities due to hypocawcemia.[18] Disodium EDTA is not approved by de FDA for any use,[17] and aww FDA-approved chewation derapy products reqwire a prescription, uh-hah-hah-hah.[19]

EDTA, which binds to cawcium, is used to awweviate de hypercawcimia dat often resuwts from band keratopady. The cawcium may den be removed from de cornea, awwowing for some increase in cwarity of vision for de patient.

Citric acid is used to soften water in soaps and waundry detergents. A common syndetic chewator is EDTA. Phosphonates are awso weww-known chewating agents. Chewators are used in water treatment programs and specificawwy in steam engineering, e.g., boiwer water treatment system: Chewant Water Treatment system. Awdough de treatment is often referred to as "softening," chewation has wittwe effect on de water's mineraw content, oder dan to make it sowubwe. What does change is de water's pH wevew, which is wowered.

Metaw chewate compounds are common components of fertiwizers to provide micronutrients. These micronutrients (manganese, iron, zinc, copper) are reqwired for de heawf of de pwants. Most fertiwizers contain phosphate sawts dat, in de absence of chewating agents, typicawwy convert dese metaw ions into insowubwe sowids dat are of no nutritionaw vawue to de pwants. EDTA is de typicaw chewating agent dat keeps dese metaw ions in a sowubwe form.[29]

The wigand forms a chewate compwex wif de substrate. Chewate compwexes are contrasted wif coordination compwexes composed of monodentate wigands, which form onwy one bond wif de centraw atom. The word chewation is derived from Greek χηλή, chēwē, meaning "cwaw"; de wigands wie around de centraw atom wike de cwaws of a wobster. The term chewate was first appwied in 1920 by Sir Giwbert T. Morgan and H. D. K. Drew, who stated: "The adjective chewate, derived from de great cwaw or chewe (Greek) of de wobster or oder crustaceans, is suggested for de cawiperwike groups which function as two associating units and fasten to de centraw atom so as to produce heterocycwic rings."[30]